The present invention relates to a method for making large span tunnels.
As is known, tunnels can be classified into the following types: for road communication (road tunnels, railway tunnels, underground railways tunnels or tubes), for hydraulic communication (aqueduct tunnels, sewage tunnels and so on) and mine tunnels or galleries.
Depending on their cross-section size, these tunnels can be further classified as small, large or very large, the most frequently used tunnel shapes being the circular and polycentric shapes for tunnels in general and rectangular shape for artificial tunnels or galleries.
The excavation can be started starting from either one or the other end of the tunnel path, or, for great length tunnels, intermediate excavation starting points may be provided, through excavation wells or windows, the excavation front advancing occurring simultaneously with the excavation and removal of the rock and building of the temporary or permanent lining.
Several excavation methods have been designed, of the so called alternating sample type; one of this method, the so-called Belgian excavation method, provides for starting excavation with a cap tunnel, which is progressively enlarged in order to coat the tunnel vault. Then there is excavated the central portion of the tunnel neck, in order to cast the tunnel piers.
In the German method, on the contrary, there are at the start excavated side tunnels, in order to cast both the piers; then there are successively made: a cap stable pit, the vault and the excavation of the rock central portion, at the end of the coating or lining operation.
A further excavation method is the so-called Italian method which comprises the steps of excavating at first starting from the tunnel neck in order to form the piers and the reverse arch and enlarging the already excavated tunnel in order to cast the remaining portion of the piers and the vault.
These known methods, however, have the drawback that they have been specifically provided for particular types of rocks: the belgian method for slightly fractured rocks, the german method for poor mechanical characteristic rocks and the italian method for very poor mechanical characteristics rocks.
Another drawback of these methods is that they do not afford the possibility of using the most recent and advanced excavation means and machines, these methods requiring moreover that reinforcing ribs and anchoring means be used, together with armoring nets and reinforced concrete.
Moreover, in the case of the excavation of large span tunnels, the ground must be preliminary consolidated for example by injection and freezing means.